Method and apparatus for correcting register faults based on changes in machine state in a multicolor printing machine

ABSTRACT

The invention relates to apparatus and a method for correcting register faults based on changes in machine state ( 4, 4′, . . . ; 5, 5′ , . . . ) in a multicolor printing machine ( 1 ) having a number of printing units ( 13, 13 ′, . . . ) with image cylinders ( 14, 14 ′, . . . ) and equipment ( 2, 2 ′, . . . ) for the digital production of color separations ( 3, 3 ′, . . . ) on the image cylinders ( 14, 14 ′, . . . ), the actions of setting up and combining the color separations ( 3, 3 ′, . . . ) being controlled in such a way that in-register prints are achieved.  
     The known methods and apparatus of this type detect register faults by means of test prints, for example, by means of register marks. As a result, changes, which lead to disturbances to the register accuracy, are registered only when a register fault is already present. By means of the invention, the occurrence of register faults as a result of dimensional changes, in particular between the printing units ( 13, 13 ′, . . . ), is to be prevented to the largest possible extent from the start.  
     This is achieved by changes in machine state ( 4, 4′, . . . ; 5, 5′ , . . . ) being registered and, on the basis of these changes in machine state ( 4, 4′, . . . ; 5, 5 ′, . . . ), the production of the color separations ( 3, 3 ′, . . . ) is changed in such a way that a disturbance to the register accuracy of the print, threatened by a change in machine state ( 4, 4′, . . . ; 5, 5 ′, . . . ), is compensated for.

[0001] The invention relates to a method of correcting register faults based on changes in machine state in a multicolor printing machine having a number of printing units with image cylinders and equipment for the digital production of color separations on the image cylinders, the actions of setting up and combining the color separations being controlled in such a way that in-register prints are achieved.

[0002] The invention also relates to an apparatus for implementing the method with a multicolor printing machine having a number of printing units with image cylinders and equipment for the digital production of color separations on the image cylinders, and at least one controller to achieve in-register prints, the actions of producing and combining the color separations being controlled.

[0003] Printing colored illustrations, in particular color images, is carried out by a number of color separations being printed over one another. These are generally the colors yellow, magenta and cyan as well as black. If required, special colors are added. By means of overprinting these colors, all color combinations can be achieved, the quality of the prints depending significantly on the in-register overprinting of the color separations. In the case of digital printing processes, for example, electrostatic printing processes, the maintenance of the register of the overprint is achieved by the image production equipment being controlled in such a way that the color separations meet one another in-register when they are transferred to a printing substrate.

[0004] A method and apparatus of the type mentioned at the 5 beginning are disclosed by U.S. Pat. No. 5,715,498. There, the correction of register faults by means of the print and the registration of register marks is proposed. Register faults are often caused by changes in machine state, for example, temperature changes lead to dimensional changes, in particular with respect to the distance between the individual printing units or between the bearings of various cylinders. The disadvantage of this technical solution is that, in the event of changes in machine state, counteractive control is always only possible for the next print, since the effect of a change in machine state is only registered when it has already caused a register fault. As a result, test prints are necessary and time is lost, which in the case of small jobs is often uneconomic and, therefore, not acceptable.

[0005] The invention is, therefore, based on the object, in a method and apparatus of the type mentioned at the beginning, of preventing the occurrence of register faults as a result of dimensional changes, in particular between the printing units or between mountings of various cylinders, to the largest possible extent from the start.

[0006] With respect to the method, according to the invention the object is achieved by changes in machine state being registered and, on the basis of these changes in machine state, the production of the color separations being changed in such a way that a disturbance to the register accuracy of the print, threatened by a change in machine state, is compensated for.

[0007] With respect to the apparatus, according to the invention, the object is achieved by at least one sensor being arranged to register changes in machine state, and the at least one controller being designed in such a way that it changes the production of the color separations in such a way that the disturbances to the register accuracy of the print, threatened by the changes in machine state, are compensated for.

[0008] The advantage of the invention is that many changes, which influence the register, are counteracted without any delay. In this way, an influence of a change in machine state can be counteracted very quickly in such a way that test prints and/or register marks are generally not necessary. It is precisely in the case of small jobs that this is of economic importance, since machine time and often also printing substrates can be saved.

[0009] However, it is, of course, possible for the measure according to the invention to be further combined with further register detection systems, control systems and regulating systems, in order to achieve monitoring and, if necessary, more exact correction, in particular in the case of further fault causes. In this case, the advantage according to the invention, that significant fault causes can be compensated for without any delay, is still maintained, and faults are avoided or at least considerably reduced. Reject prints can be avoided to the greatest possible extent in this way, and no other time-consuming control or regulating operations are needed either in order to achieve usable prints.

[0010] By means of the invention, various types of changes in machine state can be registered and taken into account. The aim is to register those changes in machine state, which have an influence on the register. These are primarily dimensional changes, in particular between the individual printing units, but also between the mountings of various cylinders, such as between image cylinders and image transfer cylinders. Such changes in spacing are caused by temperature changes and/or changes in mechanical stresses.

[0011] It is possible that the temperature of the multicolor 10 printing machine is registered at at least one location. Further possibilities are to register the mechanical stress or a change in spacing in the multicolor printing machine. Measurements are expediently made at a number of locations, the measurement locations being placed such that the changes in machine state are registered at the locations at which they exert an influence on the register, which is critical for the print quality, such as the spacing between the individual printing units. With respect to the apparatus, it is then proposed that at least one temperature sensor, at least one sensor for measuring mechanical stresses and/or at least one sensor for registering dimensional changes be provided.

[0012] For the compensation of the threatened disturbance to the register accuracy, there are various possibilities. One proposal is that a change in machine state be registered and a disturbance to the register accuracy of the print, threatened thereby, be countered by a controlled change in the production of the color separations, and then the register be regulated on the basis of registration of the printed result. In this case, counteractive control precedes the regulation, in order to minimize the magnitude of the threatened register fault from the start. Another possibility is for a change in machine state to be registered quantitatively and a disturbance to the register accuracy of the print, threatened thereby, to be compensated for, on the basis of the change in machine state, by means of stored values based on experience, which are assigned to the quantitative change in machine state.

[0013] With respect to the apparatus, it is proposed that the controller be designed in such a way that in the event of a change in machine state, it counters a disturbance to the register accuracy of the print, threatened thereby, by a controlled change in the production of the color separations, and then regulates the same on the basis of registering the printed result. When registering the printed result, a print itself, for example, the start of the print, can be registered, or it is possible that a sensor for detecting register marks is arranged in order to register the printed result. Register marks can be printed on the carrier for printing substrates or on an image-free edge of a printing substrate, for example, on a test sheet. For the other alternative with respect to compensating for the threatened disturbance to the register accuracy, it is proposed, as apparatus, that the at least one sensor be designed for the quantitative registration of changes in machine state, and the at least one controller be designed in such a way that it effects compensation for a disturbance, threatened thereby, to the register accuracy of the print by performing the control on the basis of stored values based on experience, which are assigned to the quantitative changes in machine state.

[0014] Of course, the measure according to the invention does not rule out the combination with a conventional register regulating system; it is generally an extension to one such, in order to counteract effects of a change before they manifest themselves in register faults. Provision can, therefore, be made for the controller to be provided in addition to the registration of register marks by a sensor or another register regulating system or control system. However, if other fault causes are few, then the register regulating system according to the invention can also on its own lead to the target of high register accuracy.

[0015] The invention will be explained below using an exemplary embodiment illustrated schematically in the FIGURE.

[0016] The FIGURE shows a multicolor printing machine 1, a housing 21 being illustrated as partly cut open. The printing machine illustrated has two printing units 13 and 13′, this illustration with two printing units having been selected for purposes of simplification. Normally, printing machines have four or more printing units 13, 13′, . . . . These have to be imagined as being added to the illustration in the FIGURE.

[0017] The multicolor printing machine 1 has equipment 2, 2′, for the digital production of color separations 3, 3′, . . . . The latter are applied in digital form to image cylinders 14, 14′, . . . . From the image cylinders 14, 14′, . . . , they are transferred to image transfer cylinders 15, 15′, . . . and from there to printing substrates 16. The printing substrates 16 are led through the printing machine by a carrier 17. The carrier 17 is driven by a drive roller 18 and transports the printing substrates 16 in the direction of the arrow 19.

[0018] Since the color separations 3, 3′, . . . have to arrive on the printing substrates 16 in the correct position, that is to say in-register, a controller 8, 8′, . . . is provided, which controls the positional relationships of the color separations 3, 3′, . . . to one another and to the printing substrates 16. For this purpose, the positions of the image cylinders 14, 14′, . . . , of the image transfer cylinders 15, 15′, . . . and of the carrier 17 are registered. Provided for this purpose are sensors, for example, rotary encoders 20. By means of their information, the controller 8, 8′, . . . receives the data relating to the positions of the aforementioned elements of the printing machine, which carry the image and substrate. For the purpose of allocation to a printing substrate 16, use is made of a sensor 22 for registering printing substrates 16, which is arranged at the beginning of the carrier 17. This illustration is merely an example of a register control system. The invention may also be combined with other types of register control systems, for example, one in which the production of the color separations 3, 3′, . . . is controlled by registering the time or position of register marks 12, 12′, . . . . In this case, these register marks 12, 12′, . . . are printed by the printing units 13, 13′, . . . and registered by a sensor 11.

[0019] The invention consists in changes in machine state 4, 4′, . . . ; 5, 5′, . . . being registered as early as possible, and a disturbance to the register accuracy of the print, threatened by such a change in machine state 4, 4′, . . . ; 5, 5′, . . . being counteracted immediately, before the change in machine state 4, 4′, . . . ; 5, 5′, . . . has an effect on the print quality. For this purpose, temperature sensors 9, 9′, . . . and stress sensors (sensors for measuring mechanical stresses) 10, 10′, . . . are provided. Temperature changes 4, 4′, . . . and the changes in mechanical stresses 5, 5′, . . . in the printing machine 1 are, therefore, registered immediately, and counteractive control is immediately provided, which prevents threatened disturbances to the register accuracy or at least counteracts such disturbances as early as possible.

[0020] For this purpose, the temperature sensors 9, 9′, . . . provide data about the changes in machine state 4, 4′, . . . (temperature changes), and the stress sensors 10, 10′, . . . provide data about the changes in machine state 5, 5′, . . . (changes in the mechanical stress) immediately to controllers 8, 8′, . . . . These immediately initiate changes in the production of the color separations 3, 3′, . . . , these changes being such that they are opposed to the threatened disruptions to the register accuracy.

[0021] It is particularly expedient if the temperature changes 4, 4′, . . . and the changes in the mechanical stress 5, 5′, . . . are registered quantitatively, and the change to the production of color separations 3, 3′, . . . by the controllers 8, 8′, . . . and by the equipment 2, 2′, . . . is carried out on the basis of stored values 6, 6′, . . . which are based on experience and assigned to temperatures, or by means of stored values 7, 7′, . . . which are based on experience and are assigned to mechanical stresses. However, any other type of compensation is, of course, likewise possible, for example, the registration of the direction of the threatened disturbance and counteractive control corresponding to this direction, it then being necessary for the result to be registered, for example, by means of register marks 12, 12′, . . . , and a control loop leading the correction to the end on the basis of the result.

[0022] In order to register the result of the correction, a sensor 11 for detecting register marks 12, 12′, . . . is also illustrated. These register marks 12, 12′, . . . are simultaneously printed by the printing units 13, 13′, . . . , it being possible for them to be printed on the carrier 17 on a test sheet or on the print-free edge of a substrate 16. The sensor 11 registers the position of the register marks 12, 12′, . . . and, likewise, provides this data to the controllers 8, 8′, . . . in order that the latter can check the register regulation according to the invention and, if necessary, correct it, which is particularly advantageous if still further fault causes are added. For this case, however, the invention leads to a considerable reduction in the magnitude of faults, as a result of which the register accuracy is often still within the tolerable range.

[0023] The illustration concerns one exemplary embodiment of a printing machine. Of course, this can also be constructed differently, for example, can have only four image cylinders 14, 14′, . . . , which transfer the color separations 3, 3′, . . . directly to the printing substrates 16. The incorporation of the method according to the invention and of the apparatus according to the invention depends as little on this as on the other register regulating system, to which the measure according to the invention can be seen as an addition. Parts List 1 Multicolor printing machine 2, 2’, . . . Equipment for the digital production of color separations 3, 3’, . . . Color separations 4, 4’, . . . Changes in machine state, here temperature changes 5, 5’, . . . Changes in machine state, here changes in the mechanical stress 6, 6’, . . . Stored values based on experience - assigned to temperatures 7, 7’, . . . Stored values based on experience - assigned to mechanical stresses 8, 8’, . . . Controller for achieving in-register prints 9, 9’, . . . Temperature sensor 10, 10’, . . . Stress sensor (sensor for measuring mechanical stresses) 11 Sensor for detecting register marks 12, 12’, . . . Register marks 13, 13’, . . . Printing units 14, 14’, . . . Image cylinders 15, 15’, . . . Image transfer cylinders 16 Printing substrate 17 Carrier for printing substrates 18 Drive roller 19 Arrow: transport direction 20 Rotary encoder 21 Housing 22 Sensor for registering printing substrates 

What is claimed is:
 1. A method of correcting register faults based on changes in machine state (4, 4′, . . . ; 5, 5′, . . . ) in a multicolor printing machine (1) having a number of printing units (13, 13′, . . . ) with image cylinders (14, 14′, . . . ) and equipment (2, 2′, . . . ) for the digital production of color separations (3, 3′, . . . ) on the image cylinders (14, 14′, . . . ), the actions of setting up and combining the color separations (3, 3′, . . . ) being controlled in such a way that in-register prints are achieved, wherein changes in machine state (4, 4′, . . . ; 5, 5′, . . . ) are registered and, on the basis of these changes in machine state (4, 4′, . . . ; 5, 5′, . . . ), the production of the color separations (3, 3′, . . . ) is changed in such a way that a disturbance to the register accuracy of the print, threatened by a change in machine state (4, 4′, . . . ; 5, 5′, . . . ), is compensated for.
 2. The method as claimed in claim 1 , wherein the temperature of the multicolor printing machine (1) is registered at at least one location.
 3. The method as claimed in claim 2 , wherein a mechanical stress in the multicolor printing machine (1) is registered at at least one location.
 4. The method as claimed in claim 3 , wherein dimensional changes in the multicolor printing machine (1) are registered at at least one location.
 5. The method as claimed in claim 4 , wherein a change in machine state (4, 4′, . . . ; 5, 5′, . . . ) is registered, and a disturbance to the register accuracy of the print, threatened thereby, is countered by a controlled change in the production of the color separations (3, 3′, . . . ), and then the register is regulated on the basis of registering the printed result.
 6. The method as claimed in claim 4 , wherein a change in machine state (4, 4′, . . . ; 5, 5′, . . . ) is registered quantitatively, and a disturbance to the register accuracy of the print, threatened thereby, is compensated for, on the basis of the change in machine state (4, 4′, . . . ; 5, 5′, . . . ), by means of stored values (6, 6′, . . . ; 7, 7′, . . . ) based on experience, which are assigned to the quantitative change in the machine state (4, 4′, . . . , 5, 5′, . . . ).
 7. Apparatus for implementing the method as claimed in claim 6 , with a multicolor printing machine (1) having a number of printing units (13, 13′, . . . ) with image cylinders (14, 14′, . . . ) and equipment (2, 2′, . . . ) for the digital production of color separations (3, 3′, . . . ) on the image cylinders (14, 14′, . . . ), and at least one controller (8, 8′, . . . ) for achieving in-register prints, the actions of producing and combining the color separations (3, 3′, . . . ) being controlled, wherein at least one sensor (9, 9′, . . . ; 10, 10′, . . . ) is arranged to register changes in machine state (4, 4′, . . . ; 5, 5′, . . . ), and the at least one controller (8, 8′, . . . ) is designed in such a way that it changes the production of the color separations (3, 3′, . . . ) in such a way that the disturbances to the register accuracy of the print, threatened by the changes in machine state (4, 4′, . . . ; 5, 5′, . . . ), are compensated for.
 8. The apparatus as claimed in claim 7 , wherein at least one temperature sensor (9, 9′, . . . ) is arranged in the multicolor printing machine (1).
 9. The apparatus as claimed in claim 8 , wherein at least one sensor for measuring mechanical stresses (10, 10′, . . . ) is arranged in the multicolor printing machine (1).
 10. The apparatus as claimed in claim 9 , wherein at least one sensor for registering dimensional changes is arranged in the multicolor printing machine (1).
 11. The apparatus as claimed in claim 10 , wherein the controller (8, 8′, . . . ) is designed in such a way that, in the event of a change in machine state (4, 4′, . . . ; 5, 5′, . . . ), it counters a disturbance to the register accuracy of the print, threatened thereby, by a controlled change in the production of the color separations (3, 3′, . . . ), and then regulates the same on the basis of registering the printed result.
 12. The apparatus as claimed in claim 11 , wherein in order to register the printed result, a sensor (11) is arranged to detect register marks (12, 12′, . . . ).
 13. The apparatus as claimed in claim 10 , wherein the at least one sensor (9, 9′, . . ; 10, 10′, . . . ) is designed for the quantitative registration of changes in machine state (4, 4′, . . . ; 5, 5′, . . . ), and the at least one controller (8, 8′, . . . ) is designed in such a way that it effects compensation for a disturbance, threatened thereby, to the register accuracy of the print, by performing the control on the basis of stored values (6, 6′, . . . ; 7, 7′, . . . ) based on experience, which are assigned to the quantitative changes in the machine state (4, 4′, . . . ; 5, 5′, . . . ).
 14. The apparatus as claimed in claim 13 , wherein the controller (8, 8′, . . . ) additionally makes corrections to the control of the production of the color separations (3, 3′, . . . ) on the basis of the registration of register marks (12, 12′, . . . ) by a sensor (11). 